A protein fluorescently labeled at a known molecular position can serve both as a standard for diffusional studies by FCS (fluorescent correlation spectroscopy) and FPR (fluoroscent photobleaching recovery) and as a model system for protein folding studies by FCS. For this purpose, a cystein has been incorporated at a known position in the intestinal fatty acid binding protein by Frieden and coworkers. This has been reacted with a haloacetyl derivative of fluorescein and subsequently purified through repeated dialysis and centrifugal filtering to obtain a fluorescently labeled protein of very high purity. We are currently attempting the incorporation of a rhodamine based dye in the same protein